The present invention relates to a method for generating ozone. More particularly, it relates to a method and apparatus for producing an ozone gas of sufficiently high purity and concentration to be suitable for use in the semiconductor fabrication process and similar applications. The invention also relates to methods of using the thus produced ozone.
In ozonizers of the type contemplated by the invention which are capable of producing ozone gas of high purity and concentration, stainless steel has been used as a constituent material of the parts which are to come into contact with the ozone gas. Since the semiconductor fabrication process hates the emission of fine particles and out-gases, the stainless steel is used after it is subjected to electropolishing.
In addition, with a view to producing highly concentrated ozone, an oxygen gas supplemented with nitrogen gas has been used as a feed gas.
A problem with the use of stainless steel materials in those parts of an ozonizer which are to come into contact with the ozone gas is that very small amounts of chromium (Cr) compounds are contained in the product ozone gas. A plausible reason for the generation of Cr compounds is that since oxygen (O.sub.2) gas supplemented with nitrogen (N.sub.2) gas is used as a feed gas, NOx is produced by the nitrogen within ozone generating cells and the resulting NOx reacts with the very small amount of water which is adsorbed on the surface of the stainless steel, thereby yielding nitric acid which, in turn, reacts with the stainless steel to produce Cr compounds. In addition to NOx, SOx and HF are expected to form if CF.sub.4, SF.sub.6 and NF.sub.3 are added to oxygen (O.sub.2) gas and this would be another cause of the production of Cr compounds.
The contamination of ozone gas with small amounts of Cr compounds has not been a particular concern in the semiconductor fabrication process. However, with the recent expansion of the scope of applications of ozone gas, the Cr compounds in the ozone gas used in the semiconductor fabrication process are becoming a problem and yet there are no effective means available today for dealing with the problem.